The present invention relates to a material preventing the formation of organic chlorine compounds such as polychlorinated-p-dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) (which will hereinafter be referred to as "dioxins" collectively) in an incinerator and also a method for preventing the formation of dioxins specifically. More specifically, the present invention relates to a method which makes possible to lower the dioxin concentration in an exhaust gas and in the flyash with certainty at a low cost. The present invention achieves such benefits not by using a large-scaled dioxin removing apparatus but by simply adding chemicals to the exhaust stream.
In an incinerator, a large amount of organic compounds and hydrocarbons (C.sub.n H.sub.m) are evolved. They are brought into contact with air, followed by decomposition into CO.sub.2 gas and water. If they are not brought into contact with air well, dioxin precursors are generated.
Examples of the dioxin precursors include aromatic compounds such as phenol or benzene, chlorinated aromatic compounds such as chlorophenol or chlorobenzene, chlorinated alkyl compounds, etc.
The above-exemplified dioxin precursors become sorbed to the flyash particles. "Sorbed" means adsorbed or absorbed. The difference being one of scale and distance from a surface. There is no sharp boundary between adsorption and absorption. The chemical processes of interest in dioxin formation occurs near or on a surface. Accordingly, the terms "adsorb", "adsorbed", and "adsorption" are used herein but one of ordinary skill in the art understands the extent that absorption is included in the term adsorption for such processes.
As the dioxin precursors are sorbed to the flyash particles, by catalytic action of the active sites on the flyash, the precursors are converted to dioxins. The precursors do not easily become dioxins at high temperatures of 400.degree. C. or higher even under the coexistence of the flyash. When the temperature lowers and becomes about 200.degree.-400.degree. C., however, a (catalytic) reaction as shown by the below formula, for example, occurs and these precursors convert into dioxins: ##STR1##
It is known as a method for removing the dioxins so formed, which are contained in an exhaust gas, before they are released to the environment, to inject activated carbon in the powdery form into a combustion exhaust gas line which is 120.degree.-250.degree. C. at the point before the dust collector, thereby having dioxins, which have been once formed on the flyashes in the exhaust gas treatment system, adsorbed on the activated carbon and removing them (Japanese Patent Kokai No. HEI 5-31323), incorporated by reference herein in its entirety.
The above method makes it possible to reduce the amount of dioxins in the exhaust gas, but is accompanied with the problem that the flyash contaminated with dioxins is produced. Furthermore, waste activated carbon on which dioxins have been adsorbed is generated so that another dioxin treatment becomes necessary for the disposal of the waste activated carbon.